1. Field of the Invention
The present invention relates to plant growth regulators, and more particularly, to the enhancement of production of polyisoprene rubber by stimulation of in vitro propagation of Guayule callus and shoots from a nutrient containing Guayule tissue.
2. Description of the Prior Art
Guayule is a desert shrub native to the southwestern United States and northern Mexico that produces polymeric isoprene essentially identical to that made by Hevea rubber trees in Southeast Asia. As recently as 1910 it was the source of half of the natural rubber used in the U.S. Since 1946, however, its use as a source of rubber has been all but abandoned in favor of cheaper Hevea rubber and synthetic rubbers. However, demand for natural rubber is expected to produce shortages of that material and rubber prices are expected to double by 1985. Natural rubber having lower heat hysteresis is required for many kinds of tires and amounts to about 35% of U.S. rubber use.
It is technically possible to satisfy projected demand with synthetics, but the rise in world petroleum prices has prompted the rubber industry to look for alternative sources of natural rubber. The principal, if not the only such, source is Guayule. It is conceivable that Guayule eventually could replace Hevea trees because of the susceptibility of the Hevea tree to a number of devastating diseases.
To minimize dependence on dwindling supplies of fossil fuel, attention is being directed to the production of hydrocarbons in plants such as Hevea and Guayule.
Guayule plants thrive on the dry, sandy soil of southwestern U.S. and Mexico. During World War II, extensive plantings of Guayule were carried out in California, near Indio and Salinas. Guayule normally yields one half ton of rubber per acre in cultivation when, after two years, the entire plant is harvested and processed.
Because natural Guayule has a two year harvest cycle and can only be grown in a few selected areas of warm climate regions in Arizona, California and Mexico there have been numerous attempts to speed up the growth rate and yield of valuable polyisoprene rubber contained in the plant. For example, U.S. Pat. No. 4,159,903 issued to Bauman on July 3, 1979, discloses a growth enhancement procedure wherein a 400% increase in polyisoprene rubber yields from Guayule plants is obtained by administering substituted trialkylamines to the plants. Although this method provides increased polyisoprene production, it is still limited to naturally grown Guayule grown in warm climate regions. It is therefore desirable to provide a method for propagating Guayule in vitro in an industrial setting not limited to specific climate conditions and geographical location.
Guayule has been established in vitro in tissue cultures by J. A. Bonner and B. Arreguin, Arch. Biochem., 26, 1978-186 (1950). Propagation of whole Guayule plants including roots has not been achieved. The propagation of Guayule has been limited to the establishment and maintenance of Guayule callus and shoots. Since polyisoprene is distributed throughout the entire Guayule plant including the callus and shoots, harvesting and processing according to known pyrolysis or extraction techniques of the in vitro generated callus and shoots provides a potentially useful source of polyisoprene. A further advantage of promoting in vitro propagation of Guayule callus and shoots is the potential cloning of a single species of Guayule known to have high rubber levels to produce many clones having the identical desired high rubber levels. These clones may then be planted, grown and harvested commercially. Accordingly, any method which stimulates, enhances or otherwise promotes in vitro propagation of Guayule callus and shoots is desirable.